Acoustic performance prediction of Helmholtz resonator with conical neck

There is no accurate analytical approach for the acoustic performance prediction of Helmholtz resonator with conical neck,which has broad band acoustic attenuation performance in the low frequency range.To predict the acoustic performance of the resonator accurately,a general theory model based on the one-dimensional analysis approach with acoustic length corrections is developed.The segmentation method is used to calculate the acoustic parameters for sound propagation in conical tubes.And then,an approximate formula is deduced to give accurate correction lengths for conical tubes with difierent geometries.The deviations of the resonance frequency between the transmission loss results obtained by the general theory with acoustic lengths correction and the results from the finite element method and experiments are less than 2 Hz,which is much better than the results from one-dimensional approach without corrections.The results show that the method of acoustic length correction for the conical neck greatly improved the accuracy of the one-dimensional analysis approach,and it will be quick and accurate to predict the sound attenuation property of Helmholtz resonator with conical neck.     

Inversion for the sea bottom acoustic parameters by using the group time delays and amplitude of normal mode

The group time delays and amplitudes of the normal mode are derived using the normal mode filter. The sea bottom sound speed, density and attenuation are inversely deduced by matching the group time delays and the amplitude of the normal mode. A genetic algorithm (GA) is used for optimization search in the parameter space. The inverted attenuation has the nonlinear frequency relationship a = 0.29f1.91 dB/m (the unit of frequency is kHz) in the frequency range 200 - 500 Hz. The theoretical transmission loss calculated using the deduced parameters matches the experiment data very well.     

Broadband Source Ranging in Shallow Water Using the Ω-Interference Spectrum

The concept of the waveguide invariant is revisited and a simple relationship between the range of source and the quasi-period of the acoustic interference spectrum in the frequency domain （Ω-interference spectrum） is derived. Then a method for a broadband source ranging in shallow water using a single receiver and a broadband guide source, requiring no modeling and no knowledge about the ocean environment, is proposed. The method is validated by simulation and verified with the experimental data.     

The effect of an optical pump on the absorption coefficient of magnesium-doped near-stoichiometric lithium niobate in terahertz range

The absorption coefficient of magnesium-doped near-stoichiometric lithium niobate crystal is measured by terahertz time-domain spectroscopy in a frequency range of 0.2 THz~0.9 THz at room temperature. The absorption coefficient is modulated by external optical pump fields. Experimental results show that the absorption coefficient of near-SLN：Mg crystal is approximately in a range of 22 cm- 1_35 cm- 1 in a frequency range of 0.2 THz-0.9 THz and tunable up to nearly 15%. Further theoretical analysis reveals that the variation of absorption coefficient is related to the number of light-induced carriers, domain reversal process, and OH- absorption in this crystal.     

A generalized method of converting CT image to PET linear attenuation coefficient distribution in PET/CT imaging

The accuracy of attenuation correction in positron emission tomography scanners depends mainly on deriving the reliable 511-keV linear attenuation coefficient distribution in the scanned objects. In the PET/CT system, the linear attenu- ation distribution is usually obtained from the intensities of the CT image. However, the intensities of the CT image relate to the attenuation of photons in an energy range of 40 keV-140 keV. Before implementing PET attenuation correction, the intensities of CT images must be transformed into the PET 511-keV linear attenuation coefficients. However, the CT scan parameters can affect the effective energy of CT X-ray photons and thus affect the intensities of the CT image. Therefore, for PET/CT attenuation correction, it is crucial to determine the conversion curve with a given set of CT scan parameters and convert the CT image into a PET linear attenuation coefficient distribution. A generalized method is proposed for con- verting a CT image into a PET linear attenuation coefficient distribution. Instead of some parameter-dependent phantom calibration experiments, the conversion curve is calculated directly by employing the consistency conditions to yield the most consistent attenuation map with the measured PET data. The method is evaluated with phantom experiments and small animal experiments. In phantom studies, the estimated conversion curve fits the true attenuation coefficients accurately, and accurate PET attenuation maps are obtained by the estimated conversion curves and provide nearly the same correction results as the true attenuation map. In small animal studies, a more complicated attenuation distribution of the mouse is obtained successfully to remove the attenuation artifact and improve the PET image contrast efficiently.     

Borehole guided waves in a non-Newtonian (Maxwell) fluid-saturated porous medium

<正>The property of acoustic guided waves generated in a fluid-filled borehole surrounded by a non-Newtonian (Maxwell) fluid-saturated porous formation with a permeable wall is investigated.The influence of non-Newtonian effects on acoustic guided waves such as Stoneley waves,pseudo-Rayleigh waves,flexural waves,and screw waves propagations in a fluid-filled borehole is demonstrated based on the generalized Biot-Tsiklauri model by calculating their velocity dispersion and attenuation coefficients.The corresponding acoustic waveforms illustrate their properties in time domain.The results are also compared with those based on generalized Biot's theory.The results show that the influence of non-Newtonian effect on acoustic guided wave,especially on the attenuation coefficient of guided wave propagation in borehole is noticeable.     

Correction for the diffraction loss in the ultrasonic attenuation measurement in VHF range

Two silicate glasses which have different acoustic properties and different thickness, were taken as the specimens, their frequency dependences of the longitudinal attenuation coefficients were measured in the frequency range of 50 to 300 MHz by the pulse reflection method, and the diffraction loss included in the measurement was corrected by theoretical calculation using A. O. Williams' expression. It has been shown that the measurement error of attenuation coefficient due to diffraction loss could be corrected very well by this method, regardless of the thickness of specimens.     

Frequency Dependences of Sound Attenuation and Phase Velocity in Suspensions Containing Encapsulated Microbubbles

Frequency dependences of the sound attenuation and phase velocity in an encapsulated bubbly liquid, such as ultrasound contrast agent or Levovist suspensions, at three different concentrations are studied over a bandwidth 1.5-4.5MHz by using an ultrasonic spectroscopy technique. Measurement of acoustic attenuation spectra demonstrates that the resonant frequency of the Levovist suspension is nearly 2.3-2.5MHz, and the sound attenuation enhances with the increasing concentration. With the measured sound attenuation spectra, the shear modulus and the shear viscosity are estimated to be 80 Mpa and 1.3 Pa s, respectively. The phase velocity exhibits a rapid rise with frequency smaller than 3.0 MHz, then appears to approach a frequency-independent limit above 3.0 MHz, and the change of the phase velocity over the measured frequency range is also proportional to the concentration.     

Synthesized acoustic signal characteristics of net-cage-cultured Large yellow croaker,P seudosciaena crocea

This study performs the quantitative analysis and comparison to acoustic signal characteristics of Large yellow croaker （Pseudosciaena crocea） at two different ages. The sounds were recorded from the fishes in a net-cage. Two exponential oscillation functions are built to fit the acoustic signal of the fishes. The signal characteristic of the oscillation frequency and attenuation coefficient was described quantitatively. Simulation curves of the function could fit well acoustic signals. Both the average oscillation frequency and attenuation coefficient of the fitted signals from the 13-15-month-old fishes are lower than those from the 7-8-month-old fishes. The results suggest that the oscillation frdquency and attenuation coefficient of the acoustic signal flmction may be relevant to the physical process of sound production and age characteristics of Large yellow croaker. This study may be valuable for the acoustic application to the artificial culture of the species.     

Temperature-Dependent Dielectric Characterization of Magneto-Optical Tb_3Sc_2Al_3O_(12) Crystal Investigated by Terahertz Time-Domain Spectroscopy

Terbium scandium aluminum garnet(TSAG) crystals have been widely used in magneto-optical systems. We investigate the complex refractive index of the TSAG crystal in the terahertz frequency range using terahertz(THz) time-domain spectroscopy in the temperature range 100–300 K. It is observed that the refractive index and the absorption coefficient increase with the THz frequency. The refractive index increases with the temperature.We measure the temperature coefficient of the refractive index of the TSAG crystal in the frequency range 0.4–1.4 THz. Furthermore, the loss tangent, i.e., the ratio of experimental values of the imaginary and real part of the dielectric permittivity, is found to be almost independent of frequency. TSAG is very promising for applications in THz optoelectronics because it has a high dielectric constant, low loss, and low thermal coefficient of the dielectric constant.     

利用纳米力学方法对皮肤进行超声组织定征的研究

纳米力学方法适用于具有离散特性的材料.利用纳米力学方法,得出声波在多层皮肤组织中的波动方程.分别改变黑素瘤皮肤的泊松比、黑素瘤侵袭厚度(Breslow深度)、节间距离,计算多层皮肤结构模型对垂直入射纵声波的反射系数.同时计算了声速和声衰减系数随组织参数的变化.计算结果表明,可综合利用一定频率段内反射系数最小点数、声速和衰减系数的变化来表征正常皮肤和病变皮肤.     

Modal decomposition method for acoustic impedance testing in square ducts

Accurate duct acoustic propagation models are required to predict and reduce aircraft engine noise. These models ultimately rely on measurements of the acoustic impedance to characterize candidate engine nacelle liners. This research effort increases the frequency range of normal-incidence acoustic impedance testing in square ducts by extending the standard two-microphone method (TMM), which is limited to plane wave propagation, to include higher-order modes. The modal decomposition method (MDM) presented includes four normal modes in the model of the sound field, thus increasing the bandwidth from 6.7 to 13.5 kHz for a 25.4 mm square waveguide. The MDM characterizes the test specimen for normal- and oblique-incident acoustic impedance and mode scattering coefficients. The MDM is first formulated and then applied to the measurement of the reflection coefficient matrix for a ceramic tubular specimen. The experimental results are consistent with results from the TMM for the same specimen to within the 95% confidence intervals for the TMM. The MDM results show a series of resonances for the ceramic tubular material exhibiting a monotonic decrease in the resonant peaks of the acoustic resistance with increasing frequency, resembling a rigidly-terminated viscous tube, and also evidence of mode scattering is visible at the higher frequencies.     

Acoustic characteristics of composite materials as acoustic window at oblique incidence of sound waves

This paper described a method for estimating the acoustic characteristics of composite materials at oblique incidence of sound waves. Composite materials are used as acoustic windows of SONAR to protect the internal sensors and electronic parts from water. In this study the composite material of glass reinforced plastic and polyurethane was used as the specimen. As the acoustic characteristics the velocities and attenuation coefficients of sound waves through the composite material were measured in the high frequency range. The insertion loss was also measured as a function of incident angle at 200 and 76 kHz, respectively. The attenuation coefficients in the low frequency range were estimated by interpolating the measured attenuation in the high frequency range with power-law form fitting. A four-medium layer model was proposed to estimate the insertion loss of composite materials at oblique incidence of sound waves in the low frequency range. The four-medium layer model well described the experimentally measured insertion loss at the high frequency range. It suggests that the insertion loss of the composite materials can be well estimated as a function of incident angle in the low frequency range.     

Ultrasonic shear wave properties of soft tissues and tissuelike materials

Determinations of shear wave speeds of sound and attenuation coefficients are reported for soft tissues, a silicone rubber reference material, and a gel used in manufacturing ultrasonically tissue-mimicking materials. Fresh bovine tissues were investigated, including calfskin, liver, cardiac muscle, and striated muscle. Because of the very large shear wave attenuation coefficients, reasonably accurate determinations of shear wave properties are difficult to make. The quantity measured directly was the complex reflection coefficient for shear waves at a planar interface between the sample and fused silica. Measurements were made at frequencies spanning the range 2-14 MHz. The shear wave attenuation coefficients increase with frequency and are of the order of 10(4) times the longitudinal wave attenuation coefficients. The shear wave speeds of sound also increase with frequency but are only a few percent of the longitudinal wave speeds of sound. The results are accurate enough to allow frequency dependencies to be proposed.     

Generalization of Biot’s equations with allowance for shear relaxation of a fluid

On the basis of the generalized variational principle for dissipative continuum mechanics, a system of generalized Biot’s equations is derived to describe the wave propagation in a two-phase porous permeable medium in the presence of shear relaxation in the pore-filling fluid. It was shown that the inclusion of shear viscoelasticity of the fluid leads to the appearance of two transverse modes in addition to two longitudinal modes described by the Biot theory. One of the transverse modes is an acoustic mode, whereas the other is a diffusion mode characterized by the linear frequency dependence of phase velocity and attenuation coefficient in the low-frequency region.     

阻抗复合消声器声学性能有限元预测方法及分析

为计算和分析具有复杂结构的阻抗复合式消声器的宽频消声性能,建立了一种高效声学有限元方法,给出了不同边界条件下的边界积分处理细节,得到有限元全局系数矩阵表达式,设计出计算程序框架以实现这些算法,其求解规模和计算速度与商业软件相比有优势。为计算阻抗复合式消声器的传递损失,通过阻抗管测量和数据拟合得到了吸声材料声学特性的经验公式。计算和测量了两通穿孔阻抗复合式消声器的传递损失,二者良好的吻合验证了声学有限元方法和计算程序的正确性。研究表明,插管长度影响消声器在中高频段的消声特性,右侧隔板上穿孔会消除共振峰,中高频消声性能随着出口管穿孔率的增加而提升。      

Image reconstruction in optoacoustic tomography for dispersive acoustic media

Conventional image reconstruction methods for optoacoustic tomography (OAT) assume an idealized, non-dispersive acoustic medium. However, the linear attenuation coefficient and the phase velocity of acoustic waves propagating in soft tissue depend on temporal frequency and satisfy a known dispersion law. These frequency-dependent effects are incorporated into an optoacoustic wave equation, and a corresponding reconstruction method for OAT is developed. The improvement in image fidelity that can be achieved over conventional reconstruction methods is demonstrated by use of computer-simulation studies.     

颗粒介质中声衰减的浓悬浮粒子理论及其应用

本文扼要介绍了作者建立的颗粒介质中的浓悬浮粒子理论,并将数值计算与已发表的有关实验比较,得到了满意的一致性。关于海洋沉积物的声衰减系数与频率的关系,因为不同的作者在不同的频段得到不同的关系,即高频段(几千赫以上)近似为线性关系,低频段(1千赫以下)高于一次方关系,曾产生长时期的争执,而根据本文的理论,这一争执可以得到统一。本文对颗粒参数的声学测量进行了探索,强烈地认为,除了测量颗粒介质的平均粒径之外,还要测量其第二粒径参数(如方差)才能完整描写颗粒介质的性质。本文可以作为新型颗粒参数测量仪的理论基础 关键词：     

warping变换提取单模态反演海底衰减系数

为了获得浅海海底地声模型参数,利用warping变换方法分离出单模态简正波.对于接收深度固定、定深爆炸声源情况,以简正波理论为基础定义了距离归一化的简正波传播损失,并且其随传播的距离呈线性关系,故可通过此变化规律得到声压值实部的衰减因子,进而可求得海底地声模型参数:海底衰减系数.为验证此方法的有效性,仿真了warping变换提取单模态简正波的过程,同时将warping变换提取的单模态简正波与数值计算的结果进行比较验证;并针对某次黄海试验数据进行了处理,得到在150—550 Hz频带范围内海底衰减随频率的变化规律为α=0.581f_k~(1.86)(dB/m).通过与其他学者在相同海域试验结果的对比验证,变化规律基本相同.此外不同模态间反演相同频点的衰减系数接近也较好地支撑了结果.     

Phase velocities and attenuations of shear, Lamb, and Rayleigh waves in plate-like tissues submerged in a fluid (L)

In the past several decades, the fields of ultrasound and magnetic resonance elastography have shown promising results in noninvasive estimates of mechanical properties of soft tissues. These techniques often rely on measuring shear wave velocity due to an external or internal source of force and relating the velocity to viscoelasticity of the tissue. The mathematical relationship between the measured velocity and material properties of the myocardial wall, arteries, and other organs with non-negligible boundary conditions is often complicated and computationally expensive. A simple relationship between the Lamb-Rayleigh dispersion and the shear wave dispersion is derived for both the velocity and attenuation. The relationship shows that the shear wave velocity is around 20% higher than the Lamb-Rayleigh velocity and that the shear wave attenuation is about 20% lower than the Lamb-Rayleigh attenuation. Results of numerical simulations in the frequency range 0-500 Hz are presented.